Image-forming apparatuses, e.g., copiers and printers, have in recent years been experiencing increasing diversification in their intended applications and use environments as well as demand for additional improvements in speed, image quality, and stability. For example, printers, which previously were used mainly in office environments, have also entered into use in severe environments, and the generation of stable images even under these circumstances has become critical.
Copiers and printers are also undergoing device downsizing and enhancements in energy efficiency, and magnetic monocomponent development systems that use a favorable magnetic toner are preferably used in this context.
In a magnetic monocomponent development system, development is carried out by transporting a magnetic toner into the development zone using a toner-carrying member (referred to below as a developing sleeve) that incorporates in its interior means of generating a magnetic field, e.g., a magnet roll. In addition, charge is imparted to the magnetic toner mainly by triboelectric charging brought about by rubbing between the magnetic toner and a triboelectric charge-providing member, for example, the developing sleeve. Reducing the size of the developing sleeve is an important technology in particular from the standpoint of reducing the size of the device.
When, for example, the raw materials are not satisfactorily dispersed in the magnetic toner, or during use in a severe environment, this triboelectric charging may not proceed uniformly and the magnetic toner may then become nonuniformly charged. As a result, development can occur in which only a portion of the magnetic toner is excessively charged, so-called charge-up, and various image defects may then occur.
In particular, when the developing sleeve has been downsized as referenced above, the development zone of the development nip region is narrowed and the flight of the magnetic toner from the developing sleeve is made more difficult. As a consequence, a portion of the magnetic toner is prone to remain on the developing sleeve and a trend of greater charging instability sets in.
For example, a reduction in image density can occur when charged-up toner remains on the developing sleeve, while an image defect such as fogging in the nonimage areas can be caused when the toner charge is nonuniform. Furthermore, in the case of use after standing at quiescence for a while, melt adhesion by the toner to the electrostatic latent image-bearing member may end up occurring in contact regions between the electrostatic latent image-bearing member and a member, such as the cleaning blade, that comes into contact with the electrostatic latent image-bearing member, and image defects, so-called “streaks”, may then be produced at each rotation period of the electrostatic latent image-bearing member.
To counter these problems, a large number of techniques have been introduced for controlling the dielectric characteristics—which are an index for the state of dispersion of the magnetic body in a magnetic toner—in order to stabilize the variations in the developing performance associated with changes in the environment.
For example, in Patent Literature 1, the attempt is made to lower the variation in toner charging performance associated with environmental variations by controlling the dielectric loss tangent (tan δ) in high-temperature and normal temperature zones.
While in fact a certain effect is obtained under certain prescribed conditions, in particular a high degree of raw material dispersibility at a high magnetic body content is not adequately addressed, and there is still room for improvement in particular from the standpoint of the streaks.
In addition, in order to inhibit environmental variations in the toner, Patent Literature 2 discloses a toner in which the ratio between the saturation water content HL under low-temperature, low-humidity conditions and the saturation water content HH under high-temperature, high-humidity conditions has been brought into a prescribed range.
By controlling the water content in the indicated manner, a certain effect is in fact obtained under certain prescribed conditions for the image density reproducibility and the transfer behavior. However, the charging stability is in particular not addressed for the case in which the magnetic body has been incorporated in an amount corresponding to use as a colorant, and is inadequate for obtaining the effects of the present invention.
On the other hand, in order to solve the problems associated with external additives, toners have been disclosed with a particular focus on the release of external additives (refer to Patent Literatures 3 and 4). The charging stability of magnetic toners is again not adequately addressed in these cases.
Moreover, Patent Literature 5 teaches stabilization of the development•transfer steps by controlling the total coverage ratio of the toner base particles by the external additives, and a certain effect is in fact obtained by controlling the theoretical coverage ratio, provided by calculation, for a certain prescribed toner base particle. However, the actual state of binding by external additives may be substantially different from the value calculated assuming the toner to be a sphere, and, for magnetic toners in particular, achieving the effects of the present invention without controlling the actual state of external additive binding has proven to be entirely unsatisfactory.